1. Field of the Invention
The invention relates to a device for creating a local water flow or water jet in a body of water, in such a way that bed material from the bed below said body of water can be displaced, which device comprises a jet pipe, a screw disposed rotatably in the jet pipe, and means for supplying a countertorque in the opposite direction to the torque exerted on the device through the rotation of the screw.
2. Description of the Related Art
Such a device is known from EP-A-289520. With this known device it is possible to make a trench in a water bed, in which trench a pipeline, for example, can be laid. For this purpose, the device is moved at some distance above the water bed along the path in which the trench is to be made. The rotating screw in this case supplies the desired water flow.
Since the rotating screw exerts a torque on the device, unless countermeasures were taken, the device would start to rotate uncontrollably about its axis, which is, of course, very undesirable. On account of that, blades are provided in the outlet of the jet pipe, which blades exert a countertorque on the device as a result of the water flow in the jet pipe, in such a way that the device is stabilized as regards such a rotary movement.
A great disadvantage of this device is that the torque generated and the countertorque are the same for a specific outflow rate of the water flow in the jet pipe only within a limited speed of rotation range. On variation of the distance of the device from the bottom, said outflow rate varies, and therefore so does the countertorque.
The torque generated and the countertorque are not easy to keep the same during operation of the device. Rotation of the device therefore has to be prevented by other, additional means.